The synthesis of preferred polybenzimidazole based high performance fibers requires the selective polymerization of 2,3,5,6-tetraaminotoluene (“TAT”, also known as 3-methyl-2,3,5,6-tetraaminobenzene) with various substituted and unsubstituted aromatic diacids, such as 2,5-dihydroxyterephthalic acid (“DHTA”).
has been mentioned in the literature (e.g., U.S. Pat. Nos. 3,476,590 and 3,783,137) as a comonomer in the synthesis of polybenzarenazole polymers. TAT can be used as a crosslinking comonomer using radical induced crosslinking at the methyl group. For example, in Japanese Patent Application 2005-330470 A, TAT is used as a crosslinking comonomer in the synthesis of a polybenzimidazole polymer for film applications. However, none of these references discloses a source or synthesis for TAT.
A method for preparing the 2,3,5,6-tetraaminotoluene for use in step (a) by hydrogenating 2-methyl-1,3-diamino-4,6-dinitrobenzene (II)
is disclosed in co-pending U.S. Provisional Application 61/138,602, which is by this reference incorporated in its entirety as a part hereof for all purpose.
A 1:1 complex formed between TAT and the aromatic diacid is the monomer of choice for the production of a high molecular weight polymer for high strength fiber. The ratio of diacid to tetraamine must be as close to 1:1 as possible to achieve high molecular weight. However, the oxidative instability of TAT species, including TAT, TAT salts such as TAT.nHX (n=0-4, X=Cl, Br), and the complex of TAT with aromatic diacid, during the reaction and handling processes leads to the formation of oxidation byproducts, such as 2-methyl-3,6-diiminocyclohexa-1,4-diene-1,4-diamine (Formula III),
phenazine-1,6-dimethyl-2,3,7,8-tetraamine (Formula IV) and phenazine-1,9-dimethyl-2,3,7,8-tetraamine (Formula V):
in the manner as shown by the reaction schemes set forth below:

As a consequence, the TAT:aromatic diacid ratio is far enough from 1:1 that polymer produced from it does not have high enough molecular weight for it to be suitable for the production of high-performance fiber.
In U.S. Pat. No. 5,142,021, Lysenko et al. claim a process for synthesizing a polybenzazole polymer comprising the step of reacting monomers (for example, terephthalic acid with 4,6-diaminoresorcinol, 2,5-diaminohydroquinone, 2,5-diamino-1,4-dithiobenzene or acid salts thereof) in a non-oxidizing dehydrating solvent acid solution that contains polyphosphoric acid and/or methanesulfonic acid, in the presence of a reducing agent present in a quantity of at least 0.5 weight percent of the quantity of monomers. The reducing agent is not added until the oligomerization step of the polymerization process.
However, it is important to prevent or reverse oxidation prior to the polymerization step in order to avoid side reactions causing an imbalance in the ratio of the co-monomers. It is also desirable to add as little Sn to the polymer as possible to avoid Sn contamination of the final polymer product and costly purification.
There thus remains a need for a high-yield method to prepare highly pure 2,3,5,6-tetraaminotoluene species without significant amounts of oxidation byproducts and contamination of the polymer with reducing agents or costly processes for the removal of reducing agents.